Catalytic reforming, or hydroforming, is a well established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
Platinum has been widely commercially used in recent years in the production of reforming catalysts, and platinum-on-alumina catalysts have been commercially employed in refineries for the last few decades. In the last decade, additional metallic components have been added to platinum as promoters to further improve the activity or selectivity, or both, of the basic platinum catalyst, e.g., iridium, rhenium, tin, and the like. Some catalysts possess superior activity, or selectivity, or both, as contrasted with other catalysts. Platinum-rhenium catalysts by way of example possess admirable selectivity as contrasted with platinum catalysts, selectivity being defined as the ability of the catalyst to produce high yields of C.sub.5.sup.+ liquid products with concurrent low production of normally gaseous hydrocarbons, i.e., methane and other gaseous hydrocarbons, and coke.
In a conventional process, a series of reactors constitute the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of the catalyst which receive upflow or downflow feed, and each is provided with a heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or hydrogen recycle gas, is cocurrently passed through a preheat furnace and then, usually downwardly, through a reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated from the C.sub.5.sup.+ liquid product and recycled to the process to minimize coke production.
The activity of the catalyst gradually declines due to the build-up of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene, and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by burning the coke off the catalyst at controlled conditions, this constituting an initial phase of catalyst reactivation.
Two major types of reforming are generally practiced in the multi reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series.
Various improvements have been made in such processes to improve the performance of reforming catalysts in order to reduce capital investment or improve C.sub.5.sup.+ liquid yields while improving the octane quality of naphthas and straight run gasolines. New catalysts have been developed, old catalysts have been modified, and process conditions have been altered in attempts to optimize the catalytic contribution of each charge of catalyst relative to a selected performance objective. Nonetheless, while any good commercial reforming catalyst must possess good activity, activity maintenance and selectivity to some degree, no catalyst can possess even one, much less all of these properties to the ultimate degree. Thus, one catalyst may possess relatively high activity, and relatively low selectivity and vice versa. Another may possess good selectivity, but its selectivity may be relatively low as regards another catalyst. Platinum-rhenium catalysts, among the handful of successful commercially known catalysts, maintain a rank of eminence as regards their selectivity; and they have good activity. Nonetheless, the existing world-wide shortage in the supply of high octane naphtha persists and there is little likelihood that this shortage will soon be in balance with demand. Consequently, a relatively small increase in the C.sub.5.sup.+ liquid yield can represent a large credit in a commercial reforming operation.
Variations have been made in the amount, and kind of catalysts charged to the different reforming reactors of a series to modify or change the nature of the product, or to improve C.sub.5.sup.+ liquid yield.
A response to this demand embodies a process described in said application Ser. No. 082,805, supra, wherein, in a series of reforming zones, or reactors, each of which contains a bed, or beds of catalyst, the catalyst in the leading reforming zones is constituted of supported platinum and a relatively low concentration of rhenium, and in the last reforming zone, or reactor of the series, the catalyst is constituted of platinum and a relatively high concentration of rhenium. The amount of rhenium relative to the platinum in the catalyst contained in the last reforming zone, or reactor, is in fact present in an atomic ratio of rhenium:platinum of at least about 1.5:1 and higher, and preferably the atomic ratio of rhenium:platinum ranges at least about 2:1, and higher, and more preferably from about 2:1 to about 3:1. The leading reforming zones, or reactors of the series, are provided with platinum-rhenium catalysts wherein the atomic ratio of the rhenium:platinum ranges from about 0.1:1 to about 1:1, preferably from about 0.3:1 to about 1:1. In carrying out the operation, the beds of catalyst are contacted with a hydrocarbon or naphtha feed, and hydrogen, at reforming conditions to produce a hydrocarbon, or naphtha product of improved octane, and the product is withdrawn. A further modification of this process is disclosed in pending application Ser. No. 082,804 by Swan, filed on even date with application Ser. No. 082,805.
It is known that the amount of coke produced in an operating run increases progressively from a leading reactor to a subsequent reactor, or from the first reactor to the last, or tail reactor of the series as a consequence of the different types of reactions that predominate in the several different reactors. Thus, in the first reactor of the series the metal site, or hydrogenation-dehydrogenation component of the catalyst, plays a dominant role and the predominant reaction involves the dehydrogenation of naphthenes to aromatics. This reaction proceeds at relatively low temperature, and the coke formation is relatively low. In the intermediate reactor (usually a second and third reactor), on the other hand, the acid site plays an important role in isomerizing paraffins and naphthenes, and the additional naphthenes are dehydrogenated to aromatics as in the first reactor. In both of the intermediate reactors the temperature is maintained higher than in the first reactor, and the temperature in the third reactor is maintained higher than that of the second reactor of the series. Carbon formation is higher in these reactors than in the first reactor of the series, and coke is higher in the third reactor than in the second reactor of the series. The chief reaction in the last reaction zone, or tail reactor of the series involves dehydrocyclization of paraffins, and the highest temperature is employed in this reactor. Coke formation is highest in this reactor, and the reaction is often the most difficult to control. It is also generally known that these increased levels of coke in the several reactors of the series causes considerable deactivation of the catalysts. Whereas the relationship between coke formation, and rhenium promotion to increase catalyst selectivity is not known with any degree of certainty because of the extreme complexity of these reactions, it is believed that the presence of the rhenium minimizes the adverse consequences of the increased coke levels, albeit it does not appear to minimize coke formation in any absolute sense. Nonetheless, in accordance with this invention, the concentration of the rhenium is increased in those reactors where coke formation is the greatest, but most particularly in the last reaction of the series. Thus, in one of its forms, the catalysts within the series of reactors are progressively staged with respect to the rhenium concentration, the rhenium concentration being increased from the first to the last reactor of the series such that the rhenium content of the platinum-rhenium catalysts is varied significantly to counteract the normal effects of coking.
In cyclic reforming, typically three or four reactors are arranged in series, and a swing reactor is manifolded in the unit such that it can occupy any position in the reactor train as reactors are taken out of service and the catalyst regenerated, and reactivated. Thus, in a typical catalyst regeneration, reactivation sequence in a reactor series, four reactors and a swing reactor, the swing reactor spends less than about twenty-five percent of the time in the first two reactor positions of the series, while in the remaining period the swing reactor occupies either the third or last reactor position. The last reactor of the series remains on oil about seventy percent of the time. In practicing the process wherein high rhenium is concentrated within the platinum-rhenium catalyst of the last reactor of the series, and staged in progressively higher concentration in the other reactors with highest rhenium concentration within the last reactor of the series, it may appear advantageous to substitute a high rhenium platinum-rhenium catalyst in a reactor occupying the last position of the series when this reactor is off oil for regeneration, and reactivation of the catalyst. However, placing a high rhenium platinum-rhenium catalyst in the swing reactor serves no useful purpose in the overall operation, and in fact results in significant C.sub.5.sup.+ liquid yield loss when the swing reactor occupies the first two positions as is required in conventional operations.
It is, nonetheless, the primary object of the present invention to provide a new and further-improved process, particularly one which will provide enhanced C.sub.5.sup.+ liquid yield, catalyst activity and catalyst activity maintenance credits.
A specific object is to provide a new and novel process for the operation of cyclic reforming units, notably one which will improve C.sub.5.sup.+ liquid yield, catalyst activity and catalyst activity maintenance.
These objects and others are achieved in accordance with the present invention, embodying improvements in a process for reforming naphtha, with hydrogen, in a cyclic reforming unit which contains a plurality of platinum-rhenium catalysts containing on-stream reactors in series, and a platinum-rhenium catalyst-containing swing reactor manifolded therewith which can be periodically placed in series and substituted for an on-stream reactor while the latter is removed from series for regeneration and reactivation of the catalyst. The initial and intermediate on-stream reactors of the series each contain a bed, or beds, of catalyst constituted of supported platinum and a relatively low concentration of rhenium. The last on-stream reforming reactor of the series contains a catalyst constituted of platinum and a relatively high concentration of rhenium, and the swing reactor contains multiple beds of catalysts, generally an upper (or upstream) bed, or beds, which contains catalyst constituted of supported platinum and a relatively low concentration of rhenium and generally a lower (or downstream) bed, or beds, which contains catalyst constituted of supported platinum and a relatively high concentration of rhenium. Naphtha, and hydrogen, in the series are passed initially over the bed, or beds, of the swing reactor which contains the low rhenium, platinum-rhenium catalyst and then over the bed, or beds, of the swing reactor which contains the high rhenium, platinum-rhenium catalyst. Generally, the feed is passed downwardly to contact a bed, or beds, of low rhenium, platinum-rhenium catalyst, and thereafter it contacts a bed, or beds, of high rhenium, platinum-rhenium catalyst prior to exit from the swing reactor. Preferably, the amount of rhenium relative to the platinum in the last reforming reactor, and in the lower bed of the swing reactor, is present in a weight ratio of at least about 1.5:1 and higher, more preferably from about 2:1 to about 3:1. The amount of rhenium relative to the platinum on the catalysts in the initial and intermediate on-stream reactors of the series, and upper bed of the swing reactor, are provided with platinum-rhenium catalysts wherein the weight ratio of rhenium:platinum ranges from about 0.1:1 to about 1.2:1, preferably up to about 1:1. More preferably, the rhenium:platinum ratio of the catalyst ranges from about 0.3:1 to about 1.2:1, or 1:1, and most preferably from about 0.5:1 to about 1.2:1, or 1:1. The beds of catalyst in the several reactors, inclusive of the swing reactor are serially contacted with a hydrocarbon or naphtha feed, and hydrogen, at reforming conditions the feed flowing, generally downwardly, from one reactor of the series to the next, serially through the upper and lower beds of the swing reactor, to produce a hydrocarbon, or naphtha product of improved octane, and the product is withdrawn.
Staged system credits in selectivity, catalyst activity and catalyst activity maintenance are provided by the use of a swing reactor containing an upper fixed bed of platinum-rhenium catalyst having a relatively low concentration of rhenium:platinum, and a lower fixed bed of platinum-rhenium catalyst having a relatively high concentration of rhenium:platinum. Suitably, the upper bed of the reactor contains from about 50 to about 90 percent, preferably from about 70 percent to about 85 percent of the catalyst, based on the weight of catalyst in the reactor; the balance of the catalyst (50 percent to 10 percent, preferably 30 percent to 15 percent) being contained in the lower bed, or beds, of the reactor. When the swing reactor is in the position of the first or second of the onstream reactors, the endotherm is sufficient to minimize cracking reactions in the lower zone of the reactor, thereby suppressing C.sub.5.sup.+ liquid yield loss. On the other hand, in the last and second to last onstream positions, the high concentration of rhenium in the lower bed, or beds, is beneficial in improving coke tolerance at the elevated temperatures.
These features and others will be better understood by reference to the following more detailed description of the invention, and to the drawing to which reference is made.